Photoreduction of Nitrate to HONO and NOx by Organic Matter in the Presence of Iron and Aluminum

Abstract

Nitrogen oxides (NO_y) such as NO, NO₂, and HONO control the oxidative capacity of the lower atmosphere. Studies have shown that photolysis of nitrate on atmospheric surfaces is an efficient source of nitrogen oxides through a process termed "renoxification;" however, the mechanisms responsible for this process remain poorly understood, leading to difficulties in modeling atmospheric composition. This work aims to elucidate the mechanism of NO_y formation from nitrate photolysis on model boundary layer surfaces comprised of mixtures of organic matter (citrate and Suwanee River fulvic acid) and environmentally relevant metals (e.g., Al³⁺ and Fe³⁺). Results show that in the presence of organic matter, photochemical yields of NO_y were enhanced by a factor of between 5 and 15 compared with photolysis of pure nitrate controls. Known nitrate photochemistry mechanisms are unable to explain this enhancement, suggesting that a fraction of nitrate is directly converted to NO_y by strong reductants produced photochemically from organic matter. The addition of Fe (hydr)oxides catalyzed both the reduction of NO₂ to HONO and further reduction of HONO to NO via Fe²⁺, which is formed through photoreduction of Fe-organic matter coordination complexes. In addition, this study assesses the contribution of surface acidity and visible light attenuation on the product yields. The results support a growing body of evidence that strong reductants generated photochemically via organic matter are an important and unrecognized pathway for renoxification on both soil and airborne surfaces (e.g., mineral dust and aerosols).